I. Field Of The Invention
The present invention relates to apparatus and method for directionally coupling a stripline or microstrip to a waveguide-type transmission line. The invention has application to the "slab" type of plane microwave antennas and to the control of microwave power.
II. Description Of The Prior Art
Even though the power tolerance of striplines has much increased due to the feasibility of bonding the various components, it still remains relatively modest. Therefore, with regard to high power antennas, it is sometimes necessary to use a main power distributor having waveguide design capable of tolerating high power and capable of feeding the remainder of the antenna having a stripline design. The performance of the junction between the various antenna parts in such cases is a source of power loss.
Many limitations are placed on various solutions presently used to achieve waveguide-to-stripline coupling. For example, with regard to FIG. 1, we note that the structure of a conventional waveguide-to-stripline coupler consists of a stripline 2 which includes two plates 5, 6 and a central conductor 4 located flat on one of the sides 5 of a waveguide 1. Coupling is ensured by one or more radiating slots 3 coupled to the conducting core of the microstrip 4.
Even though efficient, this form of coupling does not result in coupling coefficients in excess of 0.1, i.e., 20 dB.
In relation to FIG. 2, a conventional coaxial waveguide coupler is shown consisting of a metal bar 7 of a length nearly that of a half-wave and located within a waveguide 8. This type of coupler results in substantial coupling coefficients, but is relatively selective, with the operational frequency band being narrow. Also, this type of coupler demands a connection system 9 appropriate to the particular ultimate stripline to be used.
The present invention wholly averts the drawbacks cited above.
The present invention makes possible coupling coefficients varying over a wide range from 6 dB to more than 30 dB.
The present invention permits a wide operational frequency band, 7.5% on either side of the nominal operating frequency, without thereby modifying the characteristics.
The present invention requires no intermediary component, the coupler being used being an integral part of the stripline, and the technical connection problems being eliminated.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To that end and in conformity with the invention, a directional microwave coupler to couple microwave energy from a waveguide transmission line to a stripline transmission line is provided which comprises a longitudinal slot fashioned in a side of said waveguide; a central conductor of said stripline extending through said slot and into said waveguide; and phase-shifting means for equalizing the phase velocity of the microwaves in said conductor and the phase velocity of the microwaves in said waveguide adjacent to said conductor. Advantageously, these phase-shifting means are one or more capacitors in series with the central conductor or else one or more inductors in parallel to each other and connected between the central conductor and a wall of waveguide.
Preferably, the coupler includes a stripline wherein the central conductor is within the waveguide for a length of about one fourth of the length of the guided wave.
The invention also relates to a method for coupling a waveguide transmission line and a stripline transmission line, where this method is characterized by placing a central stripline conductor within the waveguide through a slot in a wall of the waveguide and wherein the speed of propagation of the wave in the stripline is altered, and preferably accelerated by such phase-shifting means as one or more capacitors in series with the central conductor or one or more inductors in parallel to one another and connected between the central conductor and a wall of the waveguide, so as to substantially equalize the phase velocities in the two transmission lines in the area where the central stripline conductor is within the waveguide.